The present invention relates to a microwave cooking oven and more specifically to an improved excitation system for such an oven.
Non- uniform energy distribution in the cooking cavity is a continuing problem in the design of microwave cooking ovens symptomized by hot and cold spots in the cavity. A number of different approaches to enhance uniform energy distribution by altering standing wave patterns in the cavity have been tried. One approach involves use of a so-called "mode stirrer" which typically resembles a fan with metal blades. Typically, the stirrer is located near the point where energy is coupled into the cavity. The idea is to randomize energy reflections in the cavity by introducing time varying scattering of the energy by reflection of the stirrer blades as the energy enters the cavity. Another approach involves the use of a rotating antenna in the cavity. Typically, both the mode stirrer and the rotating antenna approaches involve structures intruding into the cooking space, reducing the usable space in the cavity.
Use of stationary radiating slots beneath the food load to be heated is also known in the art. See for example U.S. Pat. Nos. 4,019,009 to Kusunoki et al; 2,704,802 to Blass et al; 3,810,248 to Risman et al. In each of these examples a waveguide structure is provided in the bottom of the cavity to couple energy from the magnetron to the slots. Such an arrangement tends to localize the energy distribution in the cavity to the vicinity of the slots and also involves a waveguide structure protruding from the bottom of the oven, again reducing the available space in the cavity. Since the slots are near field radiators, food loads are heated primarily from the bottom with hot spots in the vicinity of the specific slots. In commonly assigned U.S. Pat. No. 4,458,126 to Dills et al, a rotating upper antenna is combined with 1ower stationary slots such that the antenna pattern fills in the cold spots of the slots to provide uniform energy distribution at the cooking plane. This arrangement works very well in a full-size countertop microwave oven cavity, but involves substantial protrusion into the oven from both the top and bottom.
In view of the desire to minimize the counter space occupied by a microwave oven, there is a need for an excitation system which will provide enhanced time averaged uniformity of energy distribution in the cavity with minimum protrusion into the cooking cavity and also lending itself to a relatively low profile streamlined external oven cabinet configuration.
It is therefore an object of the present invention to provide a relatively simple efficient excitation system for a microwave oven which provides good time averaged uniformity of energy distribution in the cavity requiring no upper protrusion into the cavity and minimal space between the top and bottom walls of the cavity and the outer cabinet to provide a minimum overall height for the oven appliance.